A honeycomb structure is incorporated into an exhaust gas system or the like of a diesel engine as a trapping filter for exhaust gas, for example, as a diesel particulate filter (DPF) for trapping and removing particulate matter contained in exhaust gas from a diesel engine or the like.
Such a honeycomb structure has a structure where a plurality of cells separated and formed by porous partition walls of, for example, silicon carbide (SiC) and functioning as fluid passages are disposed in parallel with the direction of the central axis. End portions of the adjacent cells are alternately plugged (in a checkerwise pattern). That is, a cell has an open end portion on one side and a plugged end portion on the other side, and another cell adjacent to the above cell has a plugged end portion on one side and an open end portion on the other side.
By such a structure, exhaust gas can be purified by passing the exhaust gas allowed to flow into predetermined cells (inflow cells) from end portions on one side through the porous partition walls to allow the exhaust gas to flow out via the cells (outflow cells) adjacent to the inflow cells, thereby trapping particulate matter in the exhaust gas by the partition walls when the exhaust gas passes through the partition walls.
In order to continuously use such a honeycomb structure (filter) for a long period of time, the filter needs to be regenerated. That is, in order to remove the increase in pressure loss due to the particulate matter accumulating inside the filter with time, it is necessary to combust the particulate matter accumulating inside the filter for removal. Upon the filter regeneration, large thermal stress generates, and there arises a problem of causing defects such as a crack or breakage in the honeycomb structure by the thermal stress. In order to respond to a demand of improving thermal shock resistance against such thermal stress, there has been proposed a honeycomb structure having a segmentalized structure having a function of dispersing and reducing the thermal stress by unitarily bonding a plurality of honeycomb segments by a bonding material layer, and thereby the thermal shock resistance could be improved to some extent. Such a honeycomb structure of a divided structure has a structure where a plurality of honeycomb segments each having a shape constituting a part of the whole structure and a shape constituting the whole structure by being combined in a direction perpendicular to the central axis are unitarily bonded by a bonding material layer to form a honeycomb segment-bonded body having a cross-sectional shape of a predetermined shape such as a circle cut along a plane perpendicular to the central axis, followed by coating the outer peripheral face of the honeycomb segment-bonded body with a coating material.
However, since a demand for increasing in size of a filter has grown in recent years, thermal stress generating upon regeneration has increased. Accordingly, in order to inhibit the above defects, improvement in thermal shock resistance as a structure has strongly been desired. Above all, it has been desired to realize an excellent stress-relieving function and bonding strength in a bonding material layer for unitarily bonding a plurality of honeycomb segments in order to realize a honeycomb structure excellent in thermal shock resistance.
To cope with such a problem, there has been disclosed that use of a sealing agent, as a bonding material for unitarily bonding a plurality of honeycomb segments, constituted of inorganic fibers, an organic binder, an inorganic binder, and inorganic particles with the inorganic fibers having a orientation degree of 70% or more can obtain an effect in suppressing expansion and contraction in a longer axial direction of the filter (ceramic structure) and that thermal stress applied to the filter (ceramic structure) can be released even under severe use conditions (see Patent Document 1).
However, since the sealing agent disclosed in the Patent Document 1 contains an anisotropic filler such as an inorganic fiber, a Young's modulus after curing has anisotropy. That is, the above sealing agent has a low Young's modulus in a direction perpendicular to a bonded surface with a honeycomb segment and has a relatively high Young's modulus in a direction inside the bonded surface (in particular, longer axial direction). That is, with respect to the thermal stress of expansion or contraction in a longer axial direction between honeycomb segments, the sealing agent restrains honeycomb segments too much, and, as a result, there arises a problem of easily causing a crack on an end face of the resultant honeycomb structure.
In addition, since a sealing agent disclosed in the Patent Document 1 is a bonding material, and it is necessary to control properties of the bonding material by thickness, width, and length of inorganic fibers as a filler, there arises a problem of high costs.
Further, since the sealing agent disclosed in the Patent Document 1 employs inorganic fibers as a filler for the bonding material, it is not always harmless to humans.    Patent Document 1: JP-A-2002-177719